Whole-cell patch-clamp measurements were performed to investigate voltage-gated proton currents (I(PR)) in cultured murine microglia of distinct morphology and functional state. We studied I(PR) in ameboid microglia of untreated cultures, in ameboid microglia which had been activated by lipopolysaccharide, and in ramified microglia which had been exposed to astrocyte-conditioned medium. Proton currents of these three microglia populations did not differ regarding their activation threshold or the voltage dependence of steady-state activation. Moreover, pharmacological properties of I(PR) were similar: proton currents were sensitive to extracellularly applied Zn2+ or La3+, and could be abolished by each of those at a concentration of 100 microM. In the presence of extracellular Na+, I(PR) was decreased to a similar small extent due to activity of the Na+/H+ exchanger in all microglial populations. In contrast, proton currents of microglia differed between the three cell populations with respect to their current density and their time-course of activation: in comparison with untreated microglia, the current density of I(PR) was reduced by about 50% in microglia after their treatment with either lipopolysaccharide or astrocyte-conditioned medium. Moreover, I(PR) activated significantly more slowly in cells exposed to lipopolysaccharide or astrocyte-conditioned medium than in untreated cells. It can be concluded that the distinct H+ current characteristics of the three microglial populations do not correlate with the functional state of the cells.